Developments of an active matrix display device (e.g., a liquid crystal display device, a luminescent display device, and an electrophoretic display device), in which a switching element formed of a thin film transistor (TFT) that is a field-effect transistor (FET) is provided per display pixel arranged in the formed matrix have been recently actively conducted.
In these developments, focused is a technology that a TFT is produced using an oxide semiconductor film, which has high carrier mobility and give less variation between elements, in the channel formation region of the TFT, and the TFT is applied for an electronic device, or an optic device. For example, disclosed is the FET using zinc oxide (ZnO), In2O3, or In—Ga—Zn—O, as an oxide semiconductor film (see, for example, PTL 1).
As for a field-effect transistor used for a device, demanded is the one, which has high field-effect mobility, high on/off ratio, and a small absolute value of turn-on voltage.
A display device, which display in a large area, has a problem of a signal delay from lines to channels of TFTs due to resistance. As for a material of an electrode of the TFT, therefore, demanded is use of a material having low specific resistance.
When a display device using TFT is produced, a display element is laminated on top of the formed TFT. In the process performed after the formation of the TFT, therefore, a heating treatment, or an oxidizing atmosphere treatment is performed. For this reason, it is desirable to use an electrode material that does not cause deterioration through the heat treatment or oxidizing atmosphere treatment performed after the formation of the TFT.
PTL 1 discloses that a range of a carrier density of an oxide semiconductor film (e.g., an IGZO film) suitable as a channel of a semiconductive layer is 1×1011 cm−3 or greater but lower than 1×1018 cm−3, preferably 1×1014 cm−3 or greater but lower than 1×1017 cm−3, more preferably 1×1015 cm−3 or greater but lower than 1×1016 cm−3.
This is because large electric current may passed through between a source electrode and a drain electrode with no gate voltage of a transistor applied when an oxide the electron carrier density of which is 1×1018 cm−3 or greater is used as a channel of TFT, and the TFT may become a normally-on TFT. In order to produce a normally-off TFT applicable for an image display device, such as a luminescent device, it has been widely known that an oxide having an electron carrier density of lower than 1×1018 cm−3 is used as a channel of the TFT.
Proposed preferable carrier density of an oxide semiconductor layer (IGZO) used for a channel is less than 1×1017 cm−3 (see, for example, PTL 2). This is because a resulting thin film transistor may become a normally-on type if the carrier density is greater than the aforementioned range. Moreover, disclosed is a method where an oxide electroconductive material having higher carrier density than an oxide semiconductor layer as a buffer layer between a source-drain electrode layer and the oxide semiconductor layer, for the purpose of reducing contact resistance between the oxide semiconductor layer, and the source-drain electrode layer. This is because contact resistance may become large, when a material having low specific resistance (e.g. gold), which is desirable as lines or electrodes, is used as a source electrode and a drain electrode. In the case where a buffer layer is not provided, a metal having low work function, such as Al, Mo, and Ti, is typically used as an electrode in a TFT using a n-type oxide semiconductor as an active layer, in order to improve electric contact between the active layer and the source and drain electrodes. However, the metal having low work function has a problem that the metal is oxidized during a heat treatment or oxidizing atmosphere treatment performed after the formation of a field-effect transistor to increase a specific resistance.
Moreover, disclosed is to obtain a field-effect transistor, which has high mobility and reliability, by controlling an electron carrier density n (cm−3) in the range of 1018<n<1020 (see, for example, PTL 3). In the disclosed method, a structure composed of titanium (Ti)/gold (Au)/titanium (Ti) is used as source and drain electrodes.
Moreover, disclosed is to obtain a thin film transistor having a high field-effect mobility by controlling an electron carrier density n (cm−3) in the range of n≤5×1018 (see, for example, PTL 4). In the disclosed method, gold (Au) formed into a film by sputtering is used as source and drain electrodes.
Moreover, disclosed is a method, in which a semiconductor layer, and an electrode layer are formed by coating (see, for example, PTL 5). As for the method for forming the semiconductor layer of TFT using oxide semiconductor, or source-drain electrode layer, vacuum deposition, or sputtering is commonly used. In order to perform any of these methods, however, there is a problem that a complicated and expensive device is necessary. Moreover, another problem is that it is difficult to form a thin film of a large area. Accordingly, a method for forming a semiconductor layer or an electrode layer through coating is expected as a method that enables to form a film of a large area with a simple manner.
A n-type oxide semiconductor tends to exhibits higher mobility, as an electron carrier density is higher. Therefore, there is a possibility that higher on-current is attained, as an electron carrier density is higher, when the n-type oxide semiconductor is used for a field-effect transistor.
In the conventional art, however, a field-effect transistor, which has a source electrode and a drain electrode having high resistance to heat treatment and oxidizing atmosphere treatment performed after the formation of the field-effect transistor, and having low specific resistance, does not require a buffer layer, has high field-effect mobility, has high an on/off ratio, and has a small absolute value of turn-on voltage, has not been provided even in the case where the electron carrier density of the n-type oxide semiconductor is high.
Accordingly, there is currently a need for a field-effect transistor, which has a source electrode and a drain electrode having high resistance to heat treatment and oxidizing atmosphere treatment performed after the formation of the field-effect transistor, and having low specific resistance, does not require a buffer layer, has high field-effect mobility, has high an on/off ratio, and has a small absolute value of turn-on voltage, even in the case where the electron carrier density of the n-type oxide semiconductor is high.